Reveal® is a trademarked term used by the General Electric Company to refer to light sources, such as a light bulb, having enhanced red-green color contrast lighting characteristics and enhanced whiteness relative to an unmodified incandescent or halogen light source. Reveal® incandescent and halogen bulbs filter light by placing a particular type of glass (namely, glass impregnated with neodymium (Nd) oxide) in front of the light emitted by the filament which absorbs some of the yellow light. The glass impregnated with Nd oxide causes a “depression” in the yellow region of the color spectrum, so that objects viewed under this light have an enhanced color contrast, especially red and green objects which are contrasted readily by an observer, such as a person in a room of a house. The removal of some yellow light also shifts the location of the chromaticity on the CIE color diagram to a point slightly below the blackbody locus, which generally creates the impression of whiter light to most observers.
The significance of yellow light and how it impacts the perception of color is illustrated in FIGS. 1A, 1B and 1C. FIG. 1A graphs the three color matching functions, or the chromatic response of a standard observer, for XYZ chromaticity. The perceived color of an object is determined by the product of the illumination source spectrum, the reflectance spectrum of the object, and the three color matching functions. These functions are related to the response of the photoreceptors in the human eye, and can be thought of as the perception of blue (102), green (104), and red (106) light. FIG. 1b plots the product of a standard incandescent spectrum with the color matching functions for blue (132), green (134), and red (136) responses. As can be seen, the green (134) and red (136) components overlap significantly and the peaks are only separated by 34 nm. FIG. 1c plots the product of a Reveal® incandescent spectrum with the color matching functions for blue (162), green (164), and red (166) responses. As can be seen, the green (164) and red (166) components are more distinct with a peak separation of 53 nm. This allows observers to more easily distinguish reds and greens with greater contrast and results in a more saturated appearance when yellow light is suppressed.
Spectrally enhanced lighting products have enjoyed decades of commercial success. Traditional color quality metrics or conventional measurements may not reward such enhanced lighting products, yet consumers often prefer them to their unaltered counterparts. With the advent of solid-state lighting (SSL), particularly the customizability of light-emitting diode (LED) spectra, it has become apparent that current metrics are inadequate to evaluate and reflect the quality of LED products. SSL light sources, for example LEDs or OLEDs may produce light directly from the semiconductor, e.g. a blue or red or other colored LED. Or the light may be produced by conversion of the high-energy light from the SSL, e.g. a blue or violet LED, by a down-converter such as a phosphor or quantum dot or other energy converting material. The range of peak emission wavelengths for semiconductors, and the range of the peaks and widths of the emission of down-converters have been extended by recent technological development to cover a nearly continuous range throughout the visible wavelengths (about 380 nm to about 750 nm), enabling broad flexibility in tailoring the visible spectrum in order to enhance color preference for an observer. For purpose of spectral tailoring, therefore, the term light source may mean any source of visible light, e.g. the semiconductor, or LED, or OLED, or the down-converter such as a phosphor or quantum dot, or a composite of several such light sources, or a system such as a lamp or luminaire or fixture comprising such light sources.
For nearly a half-century, the color rendering index (CRI) has been the primary method of communicating the color quality of a light source. However, its effectiveness is inherently limited due to its method of calculation, particularly when dealing with spectral power distributions (SPDs) containing steep slopes versus wavelength, as often seen with LEDs. The shortcomings of CRI are well documented, and a wide variety of alternative metrics have been proposed. However, alternative color quality metrics struggle to accurately quantify consumer preference of lighting products. Houser and colleagues provide a detailed overview and comparison of a large fraction of the various color quality metrics developed in “Review of measures for light-source color rendition and considerations for a two-measure system for characterizing color rendition”, Optics Express, volume 21, #8, 10393-10411 (2013), authors K. W. Houser, M. Wei, A. David, M. R. Krames, and X. S. Shen. In general, the variety of metrics can be broken down into three broad categories pertaining to their intent and method of calculation: fidelity, discrimination, and preference. Fidelity metrics, which include CRI, quantify an absolute difference from a reference illuminant, regardless of whether the test illuminant is perceived as being better or worse, and without consideration to whether the reference illuminant is actually preferred by most observers. Discrimination metrics quantify the total area of color space that is renderable under the test illuminant, and are maximized at extreme levels of saturation and hue distortion. The many conventional color preference metrics have been developed to provide a quantitative measure of user color preference, but none provides a sufficient correlation to observer data, along with a target value to enable optimization of a light source, so that the metric can be used as a target parameter in a design optimization.
In general, it has been found that observers prefer an enhanced level of saturation, rendering colors more appealing. However, high levels of saturation, or shifts in hue, can result in unnatural rendering of colors and objects. For example, the Gamut Area Index (GAI) and the Gamut Area Scale (Qg), both of which are discrimination metrics, provide a very good correlation with observer preference up to some limit of color saturation, beyond which GAI and Qg continue to increase, while observer preference sharply declines. It therefore appears that some adjustment may be necessary to a color saturation metric such as GAI or Qg to better align it with observer preference. Furthermore, observers also tend to prefer light sources that appear whiter, driven by the color point of the illuminant relative to the Planckian (blackbody) locus, somewhat independent of the color saturation. As is generally recognized in the lighting industry, color preference cannot be adequately quantified by any single existing color metric. Several attempts have been published recently to combine two or more color metrics to better describe color preference. However, it does not appear that anyone has proposed a color preference metric that defines color preference with sufficient quantitative rigor to enable the optimization of the color preference of a light source by numerical tailoring of the spectrum. Even though the existing color preference metrics are quantitative, each is limited in some way to disqualify their use as an optimization parameter when designing a light source or a spectrum to achieve optimum color preference for a typical observer.
Some of the more well-known metrics in the color preference category include Flattery Index (Rf), Color Preference Index (CPI), and Memory Color Rendering Index (MCRI). All three of these metrics have “ideal” configurations for the chromaticity coordinates of eight to ten test color samples, and each quantifies the deviation from these target values. The flattery index was the first metric to target preference and used ten color samples with unequal weighting. However, in order to maintain similarity with CRI, the target chromaticity shifts were reduced to one-fifth of their experimental values, greatly reducing its impact. CPI maintained the experimental values for preferred chromaticity shifts, resulting in a better representation of color preference. However, it is somewhat limited in its selection of test color samples, using the same eight, unsaturated test colors as CRI. Unsaturated (pastel) test colors are incapable of evaluating the impact of a highly saturated light source. MCRI uses observers' memory to define the ideal chromaticity configuration of ten colors of familiar objects. Furthermore, none of the metrics above factor in the “whiteness”, or color point, of the test source. To this point, authors J. P. Freyssinier and M. S. Rea, in “Class A color designation for light sources used in general illumination,” Journal of Light and Visual Environment, volume 37, #2&3, pp. 46-50 (2013), recommended a series of criteria for “Class A Lighting”, which places constraints on CRI (>80), GAI (80-100), and color point (near “white” line). While these conditions define a recommended design space, they cannot be optimized to prescribe a spectrum or light source that maximizes color preference, as there is no optimal value identified, and no weighting of the three characteristics recommended.
Solid-state lighting technologies such as LEDs and LED-based devices often have superior performance when compared to incandescent lamps. This performance can be quantified by the useful lifetime of the lamp, lamp efficacy (lumens per watt), color temperature and color fidelity, and other parameters. It may be desirable to make and use an LED lighting apparatus also providing enhanced color preference qualities.
Commercial lamp types (including incandescent, halogen, and LED) employing Nd-doped glass to absorb some of the yellow light from the spectrum emitted by the light source exist, which enhance the color preference relative to their counterpart lamps without the absorption by the Nd-doped glass. GE Lighting, and some other manufacturers, has products of each of these three types. The GE Lighting products have the Reveal® brand name.
Some special formulations of phosphor for compact fluorescent (CFL), linear fluorescent (LFL), and LED lamps are known to enhance the color preference relative to their counterpart lamps that employ standard phosphors. GE Lighting has products of each of the first two types, also under the Reveal® brand name. LED light sources of the third type are known, for example in grocery applications to enhance the colors of meats, vegetables, and produce (e.g. fruit).
Each of these existing light sources has employed either Nd-doped glass, or customized phosphors that reduce the amount of yellow light emitted by the light source in order to enhance color preference. However, none of these products achieves a level of color preference exceeding that of the decades-old GE Lighting Reveal® incandescent, and the other existing products. The Nd filter in these existing light sources is typically comprised of Nd2O3-doped glass, but in other embodiments the yellow filter may be comprised of one of several other compounds of Nd or of Didymium or other compounds that preferentially absorb yellow light, embedded in various matrix host materials, for example glass, crystal, polymer, or other materials; or by some other dopant or coating on the glass that absorbs preferentially in the yellow; or by the addition of any yellow absorber to any of the optically active components of the lamp or lighting system, such as a reflector or diffuser or lens, which may be a glass or polymer or metal or any other material that accommodates the yellow absorber. The exact peak wavelength and width of the yellow absorption will vary depending on the particular Nd or rare-earth compound and host material, but many combinations of Nd, Didymium and other rare-earth compounds and host materials are suitable substitutions for the combination of Nd2O3-doped glass, as are some other yellow filters. The Nd or other yellow filter may be in the shape of a dome enclosing the light source, or may be any other geometric form enclosing the light source, such that most or all of the light in the yellow range of wavelengths passes through the filter.